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Homogeneous Reactions of Thiophenes with Transition Metals: A Modeling Approach for Elucidation of the Hydrodesulfurization Mechanism and an Effective Method for the Synthesis of Unusual Organosulfur Compounds

Abstract:

The fragment [(triphos)RhH], generated by thermolysis of (triohos)RhH3(1) in refluxing THF, reacts with thiophene (T) or benzo[b]thiophene (BT) to yield (triphos)Rh(η3-SCH═CH—CH═CH2) (2) and (triphos)Rh-{η3-S(C6H4)CH═CH2} (3), respectively [triphos = MeC(CH2PPh2)3]. Compound 2 is selectively protonated at the terminal metal-bonded carbon atom (C2) by HBF4•OEt2to give, after anion exchange, the η4-C,C,C,S-thiocrotonaldehyde complex anti-[(triphos)Rh{η4-SCHCHCH(CH3)}]BPh4(4), which reacts with CO to yield [(triphos)Rh(CO)-{η2-S═CH-CH═CH(CH3)}]BPh4(5) and thermally isomerizes to syn-[(triphos)Rh{η4-SCHCHCH(CH3)}]BPh4(6) in solution. Complex 4 also reacts with MeI by selective delivery of Me+to the sulfur atom to give, after anion exchange, [(triphos)Rh(η3-MeSCH═CH—CH═CH2)]BPh4(7). On the other hand, Ph3C+selectively attacks the C2carbon atom to yield [(triphos)Rh{η4-SCHCHCH(CH2CPh3)}]PF6(8), whose structure has been determined by X-ray diffraction. Complex 8 crystallizes in orthorhombic space group P212121(no. 19) with a = 10.834(6) Å, b = 15.012(6) Å, c = 39.902(9) Å, Z = 4, and V = 6489.66 Å3. The cation [(triphos)Rh{η4-SCHCHCH(CH2CPh3)}]+presents a distorted square pyramidal structure with one P atom occupying the apical position, while the remaining two P atoms plus the C6-S and the C7-C8 bonds occupy the basal sites; the C8 atom bears the trityl substituent. The vinylthiophenolate complex 3 is also selectively protonated at C2with HBF4•OEt2to yield [(triphos)Rh{η4-S(C6H4)-CH(CH3)}]BPh4(9), which undergoes an intramolecular hydrogen shift from carbon to sulfur slowly at room temperature and rapidly in refluxing THF to produce [(triphos)Rh{η3-HS(C6H4)CH═CH2}]BPh4(10); complex (10) is deprotonated by t-BuOK to reform 3. As in the case of 2, MeI and Ph3CPF6react with 3 by selective attack of S and C, yielding [(triphos)Rh{η3-MeS(C6H4)CH═CH2}]BPh4(11) and [(triphos)Rh{η4-S(C6H4)CH(CH2CPh3)}]PF6(12), respectively. All the rhodium complexes obtained by addition of electrophiles to 2 or 3 upon treatment with CO quantitatively transform into [(triphos)Rh(CO)2]Y (Y = BPh4, PF6), liberating the thio ligands in solution. In this manner we have prepared the new organosulfur compounds 2-n-propenyl-4-methyl-4H-1,3-dithiine, 5,5,5-triphenyl-trans-2-pentenethial, 2-ethylidenecyclohexadienethione, and 2-(3,3,3-triphenylpropylidene)cyclohexadienethione and provided a convenient synthetic method for cis-1-(methylthio)butadiene, 2-vinylthiophenol, and o-(methylthio)styrene, which have been previously made by more complicated multistep syntheses. The chemistry herein described provides useful information on the fundamental aspects of hydrodesulfurization catalysis as well as a novel entry into the synthesis of organosulfur compounds. © 1995, American Chemical Society. All rights reserved.